This invention relates generally to free-tipped axial-flow fans, and more particularly to free-tipped fans that may be utilized as automotive engine-cooling fans.
Engine-cooling fans are used in automotive vehicles to move air through a set of heat exchangers which typically includes a radiator to cool an internal combustion engine, an air-conditioner condenser, and perhaps additional heat exchangers. These fans are generally enclosed by a shroud which serves to reduce recirculation and to direct air between the heat exchangers and the fan.
The shroud plenum (that portion of the shroud adjacent to the heat exchangers) is generally rectangular and the inflow to the fan is not axisymmetric. The radiator typically has a fin-and-tube structure which contributes additional non-axisymmetric flow structures to the inflow. This lack of symmetry in the inflow causes unsteady blade loading, and the generation of acoustic tones. In addition there are several sources of broadband noise. In order to reduce both tonal and broadband noise, the fan blades are often skewed.
The fans are typically injection-molded in plastic, a material with limited mechanical properties. Plastic fans exhibit creep deflection when subject to rotational and aerodynamic loading at high temperature. This is particularly an issue when the fan is mounted downstream of the heat exchangers, where the fan operates in high-temperature air, and is further subject to radiant heat from various under-hood components. This deflection must be accounted for in the design process.
Although some engine-cooling fans have rotating tip bands, many are free-tipped. These fans are designed to have a tip gap, or running clearance, between the blade tips and the shroud barrel. This tip gap must be sufficient to allow for both manufacturing tolerances and the maximum deflection that may occur over the service life of the fan assembly. Unfortunately, large tip gaps generally result in reduced fan efficiency and increased fan noise.
Many fan assemblies using free-tipped fans are relatively low-power assemblies. These fans do not consume a large amount of electric power, nor do they make much noise. They are often designed with large tip gaps, and minimal blade skew. The resulting decrease in performance and increase in noise may not be as important as would be the case with more powerful fan assemblies.
Other fan assemblies, however, consume considerable electric power, and make objectionable noise. These assemblies must be designed to minimize noise, and maximize efficiency. To accomplish this the tip gap should be as small as possible. There is therefore a need for a fan design which minimizes the deflection of the blade tip. A problem faced by the fan designer is that the blade skew which is desirable for noise reduction often results in increased deflection.
Free-tipped fans are often designed to have a constant-radius tip shape, and to operate in a shroud barrel which is cylindrical in the area of closest clearance with the fan blades. In this case, the radial component of tip deflection is the main component of concern. However, U.S. Pat. No. 6,595,744 describes a free-tipped engine-cooling fan where the blade tips conform to a flared shroud barrel. In this case, both axial and radial tip deflection can change the size of the tip gap. Although U.S. Pat. No. 6,595,744 further describes a fan geometry which minimizes axial deflection of the blade tip for a given skew, it does not prescribe skew distributions which minimize radial deflection.